The invention relates to a method and an apparatus for performing soft handoff between cells of large differing radii, and particularly for application in systems which, due to a search window size which limits searching capabilities, can only perform handoff between cells having similar cell sizes.
CDMA (code division multiple access) communications systems are currently designed to accommodate a maximum cell size difference of about 55 km. For certain applications, such as in large service areas, it would be advantageous to be able to provision for larger cell size differences, and to be able to perform handoff between larger cells and much smaller cells.
Mobile stations search for base stations within a CDMA communications system by looking for an identification code, such as the IS-95 PN (pseudo-random number) short codes (in-phase code and quadrature code), transmitted by every base station, different sectors of different base stations sending the same short code with a different time shift, known as a PN offset, which provides a unique base station sector identifier. Mobile stations on power-up acquire their time reference from the first base station they make contact with. This time reference is delayed by the RF propagation delay between the base station and the mobile station, Before a soft handoff, a mobile station which has been informed by a serving base station of the different time shifts of the short code for target base stations knows at what time to look for the short code generated by these target base stations. The serving base station is a base station with which the mobile station has an active communications channel. The mobile station""s time reference is set according to the earliest path received from the serving base station(s). Potential target base stations are candidates for handoff which are identified, for example, on the basis of their proximity to the serving base station. The mobile station performs a search for the code over a time period known as the search window. This search window is placed according to the mobile station""s notion of time which is derived from the serving base station. After a change in serving base station, the mobile station""s search window slowly migrates so as to be centred about a time reference derived from the new base station. A problem with existing systems is that if a handoff is to be performed between two cells of large differing radii, then there is a big difference in delay between firstly, (the propagation delay between the mobile station and the target base station) and secondly (the propagation delay between the mobile station and the base station from which the mobile station has previously established a time reference) and this causes the short code of the target base station to arrive earlier or later than expected because, as indicated above, the time reference of the mobile station is derived from a different base station. If the difference in delay is sufficiently large, then the short code of the target base station will arrive outside the search window, the mobile station will fail to identify the short code, and the mobile station will by unable to perform soft handoff to the target base station.
One solution which has been proposed is to modify the mobile station such that it has a large enough search window or a search window offset so that it can find the short code regardless of the difference in delay. Problems with this are that every mobile station must be changed so as to not follow the CDMA IS-95 standard, and also that a larger search window takes a longer time to search.
It would be advantageous to be able to use existing mobile stations in environments with cells having large differing radii for interoperability reasons.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
The invention is for application in a system in which base stations each transmit a known identification code, for example a CDMA short PN code with a respective PN offset. A method is provided for allowing a mobile station to search for and find the identification code of a target base station transmitting with a particular offset and having a cell size with a large cell size difference compared to that of a serving base station, and having a large propagation delay difference between the target base station and the serving base station, without making any changes to how the mobile station operates. The method involves providing information to the mobile station which makes it look for the identification code with a different offset than the particular offset.
Preferably, the different offset is selected to have a difference from the particular offset which compensates for a difference between firstly, (the propagation delay between the mobile station and the target base station) and secondly (the propagation delay between the mobile station and the base station from which the mobile station previously established a time reference).
In the event that the identification code is a CDMA short PN code, and the respective offset is a PN offset, the information provided to the mobile station consists of a virtual PN offset for the particular target base station which is different from the target base station""s actual PN offset.
Even more generally, the invention provides a method of compensating for a large difference in propagation delay between a first signal sent with a first propagation delay and a second signal sent with a second propagation delay comprising sending information which makes a receiver search for the second signal at a time which is substantially equal to the second signal shifted by the large difference in propagation delay, within plus or minus half a search window size.